The invention relates to an anti-blocking system for a road vehicle having a hydraulic multi-circuit brake installation which can be changed over by means of an electrohydraulic shifting device from a functional state which corresponds, in the sense of a fixed determination of the front axle/rear axle braking force distribution B.sub.VA /B.sub.HA to a dimensioning for stable driving behavior up to the highest possible values of the braking Z of the vehicle, into a functional state which corresponds, likewise in the sense of a fixed determination of the ratio B.sub.VA /B.sub.VH, to a dimensioning of this ratio to a higher value of the rear axle braking force proportion.
Such an anti-blocking system is known from DE No. 34 36 223 A1 for a road vehicle which has a diagonal brake circuit division so that the two wheel brakes of the respectively mutually diagonally oppositely arranged front wheels and rear wheels are combined to form one brake circuit each. One of the two brake circuits is dynamic and is actuated by the output pressure, proportional to the force with which the driver actuates a braking device of a hydralic brake force servo unit. The other brake circuit is static and connected to an output pressure chamber of the braking device which is movably delimited by a piston acted upon one one side by the output pressure of the brake force servo unit, which produces a 1/1 transmission of the output pressure of the brake force servo unit into the static output pressure chamber of the braking device. A common outlet valve adapted to be electrically modulated, in the open position of which brake fluid can be discharged out of the rear wheel brakes to the--pressureless--tank of the auxililary pressure source, is provided for the rear wheel brakes of the vehicle in order to regulate the braking pressure in the sense of the anti-blocking regulation operation. This may be constructed as an electrically driven pump, by the intermitent--pulsed--change-over of which into the circulating operation the output pressure of the brake force servo unit can be reduced, which leads to a braking pressure reduction both in the dynamic and in the static brake circuit. An anti-blocking regulation operating according to the Select Low Principle is achieved both on the front wheel brakes and on the rear wheel brakes as a result thereof. A braking force distribution control valve constructed as a 2/2 way solenoid valve, which is provided for each of the two brake circuits, can be changed over between alternative through-flow and shut-off positions. In the through-flow positions of these valves, the rear wheel brakes are connected to the pressure output, associated with the respective brake circuit, of the braking device and are thereby also connected to the front wheel brake of this brake circuit, whereas in the shut-off positions of these valves they are shut off from the respective pressure output and the respective front wheel brake(s). By a pulsating changeover of these brake force distribution control valves with an appropriate ratio of pulse durations in which they are activated into their flow positions and their shut-off positions, different values of the front axle/rear axle brake force distribution can be obtained, while a permanent maintenance of the through-flow positions of the brake force distribution control valves corresponds to the highest possible rear axle brake force proportion, and in the case of permanent maintenance of the shut-off positions of these two valves, the braking force is built up only through the front wheel brakes.
In the brake installation according to DE No. 34 36 223 A1, the pulse duty ratio with which the braking force distribution control valves are changed over into their alternative functional positions is controlled as a function of the values of the braking slippages which occur at the front wheels and at the rear wheels, which are detected in a customary manner by processing output signals from electronic wheel velocity sensors individually associated with the vehicle wheels, so that, as further known from DE No. 33 01 948 A1, to which reference is made in this respect in the DE No. 34 36 223 A1, approximately the same utilization of friction is obtained at the--braked--front wheels and rear wheels of the vehicle, with the provison that this friction utilization, that is to say, the braking slippage, should be only relatively slightly lower at the rear wheels (by 3% to 15%, for example) than at the front wheels. This mode of braking force distribution control produces relatively good approximation to the "ideal" control in the entire range of possible vehicle decelerations, braking force distribution characterized by equality of the friction utilizlation at the front wheels and at the rear wheels, and also takes into consideration the desideratum of high driving stability within the widest possible range.
Nevertheless, such a braking force distribution, generally considered optimum and therefore also aimed at as a general rule, possesses the disadvantage, which must be regarded as very serious, that in every braking situation; that is, both in the case of a full braking where the vehicle wheels are decelerated almost to the locking limit, and also in the case of a partial braking where the vehicle wheels remain "a long way away" from the locking limit, the front wheel brakes have to perform the predominant part of the deceleration work, with the result that both the thermal and the mechanical loading of the front wheel brakes and the front wheels overall is substantially higher than that of the rear wheels. However,it is not only unfavorable here that the front wheel brakes are subjected to greater wear, which can be "compensated" by appropriately careful maintenance, but also that in driving situations, such as a prolonged pass descent in mountains, for example, in which the service brake has to be repeatedly actuated vigorously and for a relatively high proportion of the driving duration, overheating of the front wheel brakes occurs; however, with the consequence that, in the case of a vehicle having front axle/rear axle brake circuit distribution, the front axle brake circuit at first and then with high probability, the rear axle brake circuit will also fail, and in the case of a vehicle having diagonal brake circuit distribution, it will even be highly probable that both brake circuits will fail simultaneously, notwithstanding the fact that the brake installation has only been loaded in the partial braking range.
It is clear that such safety risks, which may arise in statistically significant braking situations, are unacceptable.
However, since it is to be assumed that these risks may be estimated to be lower in the case of a vehicle having front axle/rear brake circuit distribution, by virtue of the system as it were, it is therefore the object of the invention to improve an anti-blocking system of the aforementioned type for such a brake installation so that the utilization of an increased rear axle braking force proportion is possible with undiminished good driving stability.
The object is achieved according to the invention by the features to be described more fully hereinafter.
Accordingly, as long as no tendency to lock exists at the front axle, the anti-blocking regulation operates in the rear axle brake circuit according to the principle of regulating any braking slippage occuring there in relation to the braking slippage occurring at the front axle in such a manner that the rear axle braking slippage does not exceed by more than 6% a braking slippage occuring at the front axle, which may be considered adequate for a stable braking behavior of the vehicle. However, the admissible slippage differential will preferably be kept somewhat smaller, that is to say, at a value of approximately 4%, in order to ensure adequate driving stability in every case. This mode of rear axle braking slippage regulation is abandoned in favor of the "normal" anti-blocking regulation when and as long as the regulation is also effective at the front axle, while in this case the relative braking slippage both at the front axle and at the rear axle, referred in this case to a reference velocity characteristic of the vehicle speed adapted to be determined by known criteria, is kept within a range of values restricted by a slippage threshold which ensures adequate driving stability.
The regulation algorithm according to this invention ensures high sensitivity of the regulation and leads to good utilization of the braking forces which can be exerted by way of the rear axle.
According to other features of this invention, alternative constructions of the electrohydraulic shifting device may be used, by means of which the brake installation is controllable at the commencement of braking into that functional state which corresponds to the braking force distribution with the higher rear axle braking force proportion. A pressure reducer provided for this purpose according to this invention may be readily integrated into a braking device of the brake installation constructed as a tandem master cylinder.
According to still further features of this invention, measures may be adopted alternatively as also optionally in combination, for obtaining a reference velocity by means of which the rear wheel braking slippage is determined and regulated when and so long as the regulation is only effective on the rear axle. In this context, particularly good sensitivity of the regulation, which promotes the stability of the braking behavior of the vehicle, is obtained by the particular reference signal gathering provided according to this invention.
Good sensitivity may also be achieved by utilizing as reference velocity the wheel circumference velocity of the respective fastest-rotating front wheel.
In the case of a construction of the brake installation as disclosed herein, it is possible for the installed braking force distribution to be changed over by utilizing differently large effective piston surfaces of the wheel brake cylinders, by "disconnecting" or "connecting" a T-partial circuit of the rear axle brake circuit, for example.
According to still further features of the present invention, a particularly good sensitivity of reaction on the part of the anti-locking regulation may be achieved as long as the latter is effective only on the rear axle, and ensure thereby that the maximum transmissible braking forces can be utilized as soon as the regulation takes effect on the front axle.